This invention relates to a novel interrelationship of recuperators and regenerators to provide a hot air blast with, if desired, a higher temperature but more importantly in a more economical manner to improve the operation of a blast furnace or the like.
The use of a hot air blast in a blast furnace is a necessary and accepted practice for increasing the iron-making capacity while reducing the amount of coke required per ton of iron produced. At the present time, usually three or four blast furnace stoves are used alternately to preheat an air blast before delivery through a hot blast main, bustle pipe and tuyeres into the bottom part of a blast furnace. Heating of the air blast not only intensifies and speeds up the burning of coke at the tuyeres but also reduces the amount of coke required for the smelting operation in the blast furnace. The temperature of the air blast has increased throughout the history of the blast furnace by increasing the capacity, e.g., size of the traditional blast furnace stoves, or increased heating rate, or checker design. However, the efficiency of the antiquated stove design is rapidly becoming unacceptable due to increased costs and decreasing supplies of energy.
The blast furnace stoves used today embody the same basic design as originally developed more than 100 years ago. The size of each stove is approximately 25 feet in diameter and 120 feet high, although more recently-built stoves are each about 30 feet in diameter and 150 feet high. The blast furnace stoves have a brick lining enclosed in a circular steel shell with a flat bottom and a dome-shaped top. In each stove there is a vertical passageway forming a combustion chamber wherein clean blast furnace gas is burned. The combustion chamber extends from a point near the bottom of the stove to the bottom portion of the dome where hot products of combustion pass across a breast wall into a larger vertical regenerator chamber which is substantially filled with superimposed courses of checkerbricks. The filling of checkerbricks which extends from the dome to the bottom part of the stove, extracts heat from the hot products of combustion before being discharged from the bottom of the stove. The checkerwork contains a multiplicity of vertical passageways to conduct the hot products of combustion which move downwardly through the regenerator section. The temperature of the exit gases is a measure of the efficiency of the stove. The heavy weight of modern checkerwork requires metallic supports, typically a metallic grid, in the bottom of the regenerator chamber of the stove to support the checkerwork. The temperature to which the grid can be heated is limited to about 650.degree. F. because of the high loading on the grid and the grid material. A slightly higher temperature limit is possible with properly suited alloy steel. Each layer or course of refractory checkerbrick supports the layer or course above it and, therefore, the height of the stove is a determining factor for the total load that must be sustained by the refractory at the bottom of the regenerator chamber as well as the metallic grid. Because of this stove design, any attempt to increase the temperature of the hot blast requires either a higher dome temperature or a higher checker temperature or both. The refractories now used in the dome and the upper section of the stove are limited to a working temperature of about 2400.degree. F. More expensive and generally less stable refractories must be used to achieve a significant temperature increase.
The combustion chamber location and refractory lining are the source of another major problem in blast furnace stoves. The breast wall of the combustion chamber within the burner area must operate at a temperature at or above 2500.degree. F. on the burner side while at the grid side, the gas exit temperature cannot exceed approximately 650.degree. F. This extreme temperature differential on opposite sides of the same breast wall area causes very high thermal stresses in the refractory with the attending result of high maintenance cost. The high temperature thermal cycling in this area increases maintenance and frequently results in thermally-caused cracks in the wall, permitting short-circuiting of the hot products of combustion.
Irrespective of whether it is desired to increase the temperature of the hot air blast, the efficiency of the blast furnace stoves can be increased by reducing the temperature of waste gases delivered from the stove. In present-day systems of blast furnace stoves, the waste gases of combustion are discharged at a temperature of approximately 650.degree. F. and sometimes even 750.degree. F. An overall increase in the thermal efficiency can be significantly achieved by reducing the temperature of waste gases to, for example, approximately 300.degree. F.